In tire rubber compositions used for treads, sidewalls, base treads, and the like, carbon black has been widely used as a filler in order to give reinforcement, but silica has been increasingly used due to the recent demand for fuel economy and concerns about exhaustion of petroleum resources. However, since silica has a hydrophilic silanol group on its surface, silica has lower affinity for rubber (in particular, natural rubber, butadiene rubber, styrene butadiene rubber, and the like rubbers often used for tires) than that of carbon black, and silica is often inferior to carbon black in abrasion resistance and mechanical strength (tensile strength, elongation at break, crack resistance, flex crack growth resistance, tear strength, and the like performances).
For example, rubber compositions containing carbon black with a rubber component that is obtained by blending natural rubber which shows excellent tensile strength and tear strength, butadiene rubber which improves flex crack growth resistance, and the like rubbers have been conventionally used for sidewalls and base treads of tires. If a large proportion of or otherwise all of the carbon black is replaced with silica, crack resistance and flex crack growth resistance problematically decrease. This is presumably because, for example, the silica dispersibility tends to be low compared with that of carbon black, and breaking energy (tensile strength×elongation at break) is less likely to be sufficiently secured. In case of a rubber composition for a tread, such problems as reduction in abrasion resistance arise when silica is used in place of carbon black.
In order to solve these problems, methods with a silane coupling agent or with finely-divided silica having a high reinforcing effect are considered. However, finely-divided silica is usually very difficult to disperse in a rubber composition. Accordingly, the silica is not well dispersed, agglomerates remain, and abrasion resistance and mechanical strength may not be much improved or may be further deteriorated.
Silane coupling agents such as bis(3-triethoxysilylpropyl)disulfide and bis(3-triethoxysilylpropyl)tetrasulfide, which are widely used in conventional tire rubber compositions, greatly improve the silica dispersibility and impart good mechanical properties to the rubber compositions. However, dispersion of finely-divided silica requires a large amount of a silane coupling agent, resulting in a great increase in cost, and good dispersion of the silica may not be obtained even if the silane coupling agent is sufficiently added. In addition, if a rubber composition is kneaded at a high temperature for sufficient reaction between finely-divided silica and such a coupling agent, the rubber composition tends to gel or scorch.
Mercapto group-containing silane coupling agents are proposed as coupling agents that are more highly reactive than the conventionally-used coupling agents. The silane coupling agents are highly reactive and thus show high performance. However, the scorch time is very short, and the silane coupling agents have difficulty in practical use in the tire industry and are hardly used now.
Patent Document 1 discloses a tire rubber composition that contains silica and improves wet grip performance without deteriorating rolling resistance and abrasion resistance. Patent Documents 2 and 3 disclose rubber compositions for a sidewall that contain silica and have good flex crack growth resistance and tear strength. However, these rubber compositions are further required to improve rolling resistance, abrasion resistance, and wet grip performance in good balance, and to improve rolling resistance, flex crack growth resistance, tear strength, and crack resistance in good balance.    Patent Document 1: JP 2008-31244 A    Patent Document 2: JP 2006-70093 A    Patent Document 3: JP 2007-56205 A